In the field of optical communication, a polarizing glass sheet is used for a polarization-dependent optical isolator. The optical isolator is a device configured to transmit oscillation light from a laser diode (LD) or the like only in one direction and block reflected return light, and includes an optical element in which a Faraday rotator (e.g., a garnet single crystal film) is held by two polarizing glass sheets, and a magnetic member (magnet) configured to apply a magnetic field to the optical element.
In order to meet recent market needs, for the purposes of supporting downsizing and reducing cost by simplifying steps, there has been adopted a manufacturing method for the optical isolator involving bonding a Faraday rotator measuring, for example, about 10 mm square and polarizing glass sheets each having substantially the same dimensions as those of the Faraday rotator to each other, to thereby manufacture a large optical element (optical element base material), and cutting the optical element into chips (optical elements) each measuring from 0.5 mm square to 2.0 mm square.
The polarizing glass sheet has a structure in which stretched metal particles of silver, copper, or the like are dispersed in an aligned manner in a glass matrix. It has been known that when light that vibrates in various directions enters the polarizing glass sheet, a transmitted quantity of the light may vary depending on the vibration direction. For example, light that vibrates in a direction parallel to the stretching direction of the stretched metal particles is easily absorbed by the stretched metal particles, and the transmitted quantity thereof becomes a minimum. Meanwhile, light that vibrates in a direction perpendicular to the stretching direction of the stretched metal particles is not easily absorbed by the stretched metal particles, and the transmitted quantity thereof becomes maximum. The ratio between the maximum quantity and the minimum quantity of light that is transmitted through the polarizing glass sheet is called an extinction ratio. As the extinction ratio increases, the polarizing glass sheet is more excellent in characteristics.
In general, the polarizing glass sheet is manufactured as described below. First, a raw material batch containing a metal element, such as silver or copper, and a halogen element is prepared, and the raw material batch is melted and formed, to thereby manufacture a glass sheet. The obtained glass sheet is subjected to heating treatment to precipitate metal halide particles in the glass sheet, to thereby provide a glass preform sheet. The glass preform sheet is subjected to down-drawing while being heated, to thereby provide a glass member having stretched metal halide particles dispersed in an aligned manner in a glass matrix. Further, the glass member is subjected to reduction treatment to reduce the stretched metal halide particles into stretched metal particles, to thereby provide a polarizing glass sheet (for example, Patent Literature 1).
In the above-mentioned manufacturing method, an angle variation (hereinafter sometimes referred to as “polarizing axis deviation”) is liable to occur between respective stretched metal halide particles. Specifically, an angle of the stretched metal halide particles is liable to tilt gradually from a direction parallel to a down-drawing direction from a center portion to both end portions in a width direction of the glass member obtained by down-drawing of the glass preform sheet. When the polarizing axis deviation increases, an in-plane variation of the extinction ratio of the polarizing glass sheet is liable to increase. Therefore, when a large optical element is manufactured and cut as described above, a variation in extinction ratio between the respective chips increases, and in some cases, a defective chip that has not reached a desired extinction ratio is generated, with the result that there is a risk in that a yield may decrease.
In order to suppress the polarizing axis deviation in the polarizing glass sheet, various methods have been proposed. In Patent Literature 2, there is disclosed a method involving repeating a plurality of times the step of deforming the glass preform sheet in a predetermined direction by applying a load under a state in which the glass preform sheet is heated to be softened from a direction different by 180°. Further, in Patent Literature 3, there is disclosed a method involving appropriately adjusting the movement speed of the glass preform sheet and the take-up speed of the stretched glass sheet.